One issue that must be handled by all cellular communications networks is mobility of mobile devices. In particular, a cellular communications network must enable handovers of mobile devices between cells within the same Radio Access Network (RAN) as well as enable handover of mobile terminals between different RANs. A common mobility issue is mobility connection failures, i.e., connection failures during or shortly after the handover process. In order to address this mobility issue, according to discussions in the 3rd Generation Partnership Project (3GPP), a mobile device, which is referred to as a User Equipment or User Element (UE), is required to transmit a failure report to the cellular communications network whenever a mobility connection failure occurs. The failure report will then be used by a Mobility Robustness Optimization (MRO) function of the cellular communications network to optimize mobility settings, or mobility parameters, that control handovers within the cellular communications network.
With respect to Inter-Radio Access Technology (IRAT) handovers (HOs), 3GPP RAN Working Group 3 (WG3) has identified multiple high priority scenarios that present mobility issues and therefore need to be addressed. As illustrated in FIGS. 1A and 1B, an IRAT HO is a handover of a UE 10 between a cell 12 served by a base station (BS) 14 in a RAN operating according to one Radio Access Technology (RAT) (e.g., an enhanced Node B (eNB) in a RAN of a 4G Long Term Evolution (LTE) cellular communications network) and a cell 16 served by a base station 18 in another RAN operating according to another RAT (e.g., a Node B in a Universal Terrestrial Radio Access Network (UTRAN) of a 3G Universal Mobile Telecommunications System (UMTS) cellular communications network). In particular, the scenarios identified by 3GPP RAN WG3 are:                Scenario 1: A mobility connection failure, specifically a Radio Link Failure (RLF), while in an LTE RAN or during a HO from the LTE RAN to a 2G/3G RAN (e.g., a UTRAN) followed by a reconnection to the 2G/3G RAN (i.e., a too late HO from an LTE RAN to a 2G/3G RAN).        Scenario 2: A mobility failure during or after a HO from a 2G/3G RAN (e.g., a UTRAN) to an LTE RAN followed by a reconnection back to the 2G/3G RAN (i.e., the source RAT). The reconnection may be to the source cell for the HO or a different cell in the 2G/3G RAN. This is referred to herein as a too early HO from a 2G/3G RAN to an LTE RAN.                    Scenario 2a: A handover failure (HOF) during the HO from the 2G/3G RAN to the LTE RAN (i.e., a HOF during a Random Access Channel (RACH) attempt in the LTE RAN) followed by the reconnection back to the 2G/3G RAN.            Scenario 2b: An RLF in the LTE RAN shortly after the HO from the 2G/3G RAN to the LTE RAN (i.e., an RLF after successful RACH in the LTE RAN) followed by the reconnection back to the 2G/3G RAN.                        
Triggering of an IRAT HO from a cell in an LTE RAN to a cell in a UTRAN is controlled by mobility parameters in the LTE RAN associated with both Reference Signal Received Power (RSRP) and Reference Signal Received Quality (RSRQ) measurement types. These mobility parameters in the LTE RAN form a HO threshold, which is referred to herein as ho_thresh_lte. One way to optimize Scenario 1 (i.e., too late HOs from LTE RAN to 2G/3G RAN, e.g., a UTRAN) is to increase the value of ho_thresh_lte in order to trigger HOs from the LTE RAN to the 2G/3G RAN earlier. However, doing so may increase the number of unnecessary HOs, i.e., HOs from the LTE RAN to the 2G/3G RAN even when the coverage of the LTE RAN is sufficient to maintain the connection. This tradeoff between decreasing the number of too late HOs and increasing the number of unnecessary HOs is illustrated in FIG. 2. An MRO algorithm should take this tradeoff into account to increase or decrease ho_thresh_lte.
Triggering of an IRAT HO from a cell in a UTRAN to a cell in an LTE RAN is controlled by other mobility parameters in the UTRAN associated with both RSRP and RSRQ measurement types. These mobility parameters in the UTRAN form a HO threshold, which is referred to herein as ho_thresh_utran. One way to optimize Scenario 2 (i.e., too early HOs from 2G/3G RAN, e.g., a UTRAN or Global System for Mobile Communications (GSM) Enhanced Data Rates for Global Evolution (EDGE) RAN (GERAN), to an LTE RAN) is to increase the value of ho_thresh_utran in order to only trigger a HO to the LTE RAN when the signal from the LTE RAN is strong enough to retain the connection. However, doing so may unnecessarily increase time in the UTRAN if ho_thresh_utran is set too high such that the UE 10 remains in the UTRAN even when the coverage of the LTE RAN is sufficient to retain a connection with the UE 10. This tradeoff is illustrated in FIG. 3 and should be taken into account by an MRO algorithm when increasing or decreasing the ho_thresh_utran.
The occurrence of too late and unnecessary HOs from an LTE RAN to a 2G/3G RAN are to be detected via RLF reports and unnecessary HO indicators. Procedures to be performed upon RLF detection are standardized in 3GPP Technical Specification (TS) 36.311 section 5.3.11.3. At the UE 10, when an RLF is detected, various information is stored in an RLF report as illustrated in FIG. 4. In the case where the RLF is followed by a Radio Resource Control (RRC) connection re-establishment procedure, the UE 10 sets the reestablishmentCellId in the RLF report to a global cell identity of the selected cell. Additional information to be reported in support of the MRO function particularly with respect to IRAT HOs is currently under discussion in 3GPP RAN3. At this point, the discussions are initially progressing towards a decision about how to make the RLF reports available to the different RATs as explained below.
Different solutions to making RLF reports associated with IRAT HOs available to the different RATs running MRO algorithms have been proposed. These solutions are described in 3GPP Written Contribution R3-120390, which is entitled “IRAT MRO way forward” and was presented in 3GPP Meeting R3-75 which was held from Feb. 6, 2012 through Feb. 10, 2012 in Dresden, Germany. As described in 3GPP Written Contribution R3-120390 and discussed below, there are four different solutions.
Solution 1: The first solution is reporting the RLF when returning to the LTE RAN. More specifically, for both Scenario 1 and Scenario 2 discussed above, when the UE reconnects to the 2G/3G RAN after the mobility failure, the UE stores the necessary information for the corresponding failure report. Then, when the UE is back in the LTE RAN, the failure information is transmitted to the LTE RAN as, for example, an RLF report. The base station in the LTE RAN that obtains the RLF report from the UE forwards the RLF to the base station that serves the cell where the corresponding mobility connection failure occurred via appropriate signaling (e.g., X2 or S1 signaling for Scenarios 1 and 2b and RAN Information Message (RIM) to the Radio Network Controller (RNC) of the base station serving the cell in the 2G/3G RAN before the IRAT HO for Scenario 2a).
Solution 1 for Scenario 1 is illustrated in FIG. 5. As illustrated, a UE experiences an RLF in the LTE RAN. After the RLF, the UE connects to Cell Y in the 3G RAN and stores the RLF report. Subsequently, when the UE reconnects to the LTE RAN by, in this example, an IRAT HO from Cell Y in the 3G RAN to Cell B in the LTE RAN, the UE sends the RLF report to the base station corresponding to Cell B in the LTE RAN. The base station corresponding to Cell B sends the RLF report to the base station corresponding to Cell A where the RLF occurred. The MRO function of the base station for Cell A determines that an amount of time that the UE was connected to Cell A before the RLF (Δt) is greater than a predefined minimum amount of time (t_min) and, as such, the RLF was due to a too late IRAT HO from the LTE RAN to the 3G RAN.
Solution 1 for Scenario 2a is illustrated in FIG. 6. After a HO failure (i.e., unsuccessful RACH attempts) during an IRAT HO from Cell X of the 3G RAN to Cell A of the LTE RAN, the UE reconnects to Cell Y of the 3G RAN. Subsequently, when the UE reconnects to the LTE RAN by, in this example, an IRAT HO from Cell Y in the 3G RAN to Cell B in the LTE RAN, the UE sends the RLF report to the base station corresponding to Cell B in the LTE RAN. The base station corresponding to Cell B in the LTE RAN determines that the mobility failure is an IRAT HOF from Cell X in the 3G RAN and, as such, sends the RLF report to the RNC for the base station corresponding to Cell X of the 3G RAN via a RIM.
Solution 1 for Scenario 2b is illustrated in FIG. 7. Shortly after an IRAT HO from Cell X of the 3G RAN to Cell A of the LTE RAN, the UE experiences an RLF. After the RLF, the UE reconnects to Cell Y of the 3G RAN. Subsequently, when the UE reconnects to the LTE RAN by, in this example, an IRAT HO from Cell Y in the 3G RAN to Cell B in the LTE RAN, the UE sends the RLF report to the base station corresponding to Cell B in the LTE RAN. The base station corresponding to Cell B in the LTE RAN determines that the mobility failure is an RLF shortly after the IRAT HO from Cell X in the 3G RAN to Cell A in the LTE RAN (i.e., the IRAT is a too early IRAT) and, as such, sends the RLF report to the RNC for the base station corresponding to Cell X of the 3G RAN via a RIM. In addition, the base station corresponding to Cell B may send the RLF report to the base station corresponding to Cell A in the LTE RAN where the RLF occurred via suitable signaling (e.g., X2 or S1).
Solution 2: The second solution is reporting the failure to the 2G/3G RAN and/or the LTE RAN where the UE reconnects after the mobility failure. More specifically, Solution 2 for Scenario 1 is illustrated in FIG. 8. As illustrated, the UE experiences an RLF in Cell A of the LTE RAN due to a too late HO to the 3G RAN. After the RLF, the UE stores the RLF report and sends the RLF report to the 3G RAN upon reconnecting to Cell Y of the 3G RAN. The RNC of the base station corresponding to Cell Y of the 3G RAN determines that the RLF report is the result of a too late IRAT HO from Cell A of the LTE RAN and therefore sends the RLF report to the base station corresponding to Cell A of the LTE RAN via a RIM.
Solution 2 for Scenario 2a is illustrated in FIG. 9. As illustrated, after a HO failure (i.e., unsuccessful RACH attempts) during an IRAT HO from Cell X of the 3G RAN to Cell A of the LTE RAN, the UE stores a corresponding RLF report and sends the RLF report to the 3G RAN upon reconnecting to Cell Y of the 3G RAN. The RNC of the base station corresponding to Cell Y of the 3G RAN determines that the RLF report is the result of a too early IRAT HO from Cell X of the 3G RAN to Cell A of the LTE RAN. In addition, the RNC may send the RLF report to the base station corresponding to Cell A of the LTE RAN via a RIM. Notably, the RLF report can be used by the MRO function of the RNC and/or an MRO function of the base station corresponding to Cell A of the LTE RAN. If the UE reconnects to the LTE RAN after the failure and the RLF report is not yet reported to the LTE RAN, the UE may send the RLF report to a serving base station in the LTE RAN. The serving base station can then forward the RLF report to the RNC of the base station corresponding to Cell X in the 3G RAN via a RIM and, if desired, send the RLF report to the base station corresponding to Cell A in the LTE RAN.
Solution 2 for Scenario 2b is illustrated in FIG. 10. As illustrated, shortly after an IRAT HO from Cell X of the 3G RAN to Cell A of the LTE RAN, the UE experiences an RLF. After the RLF, the UE stores an RLF report and sends the RLF report to the 3G RAN upon reconnecting to Cell Y of the 3G RAN. The RNC of the base station corresponding to Cell Y of the 3G RAN determines that the RLF report is the result of a too early IRAT HO from Cell X of the 3G RAN to Cell A of the LTE RAN. In addition, the RNC may send the RLF report to the base station corresponding to Cell A of the LTE RAN via a RIM. Notably, the RLF report can be used by the MRO function of the RNC and/or an MRO function of the base station corresponding to Cell A of the LTE RAN. If the UE reconnects to the LTE RAN after the failure and the RLF report is not yet reported to the LTE RAN, the UE may send the RLF report to a serving base station in the LTE RAN. The serving base station can then forward the RLF report to the RNC of the base station corresponding to Cell X in the 3G RAN via a RIM and, if desired, send the RLF report to the base station corresponding to Cell A in the LTE RAN.
Solution 3: The third solution is reporting the RLF to the RAT where the failure occurred and reporting the HO failure in the RAT of the cell in which the HO command was received. More specifically, Solution 3 for Scenario 1 is illustrated in FIG. 11. Notably, Solution 3 for Scenario 1 is the same as Solution 1 for Scenario 1. As illustrated, a UE experiences an RLF in the LTE RAN. After the RLF, the UE connects to Cell Y in the 3G RAN and stores the RLF report. Subsequently, when the UE reconnects to the LTE RAN by, in this example, an IRAT HO from Cell Yin the 3G RAN to Cell B in the LTE RAN, the UE sends the RLF report to the base station corresponding to Cell B in the LTE RAN. The base station corresponding to Cell B sends the RLF report to the base station corresponding to Cell A where the RLF occurred. The MRO function of the base station for Cell A determines that an amount of time that the UE was connected to Cell A before the RLF (Δt) is greater than a predefined minimum amount of time (t_min) and, as such, the RLF was due to a too late IRAT HO from the LTE RAN to the 3G RAN.
Solution 3 for Scenario 2a is illustrated in FIG. 12. As illustrated, after a HO failure (i.e., unsuccessful RACH attempts) during an IRAT HO from Cell X of the 3G RAN to Cell A of the LTE RAN, the UE stores a corresponding RLF report and sends the RLF report to the 3G RAN upon reconnecting to Cell Y of the 3G RAN. The RNC of the base station corresponding to Cell Y of the 3G RAN determines that the RLF report is the result of a too early IRAT HO from Cell X of the 3G RAN to Cell A of the LTE RAN. If desired, the RNC sends the RLF report to the base station corresponding to Cell A of the LTE RAN via a RIM.
Solution 3 for Scenario 2b is illustrated in FIG. 13. Notably, Solution 3 for Scenario 2b is the same as Solution 1 for Scenario 2b. Shortly after an IRAT HO from Cell X of the 3G RAN to Cell A of the LTE RAN, the UE experiences an RLF. After the RLF, the UE reconnects to Cell Y of the 3G RAN. Subsequently, when the UE reconnects to the LTE RAN by, in this example, an IRAT HO from Cell Y in the 3G RAN to Cell B in the LTE RAN, the UE sends the RLF report to the base station corresponding to Cell B in the LTE RAN. The base station corresponding to Cell B in the LTE RAN determines that the mobility failure is an RLF shortly after the IRAT HO from Cell X in the 3G RAN to Cell A in the LTE RAN (i.e., the IRAT is a too early IRAT) and, as such, sends the RLF report to the RNC for the base station corresponding to Cell X of the 3G RAN via a RIM. In addition, the base station corresponding to Cell B may send the RLF report to the base station corresponding to Cell A in the LTE RAN where the RLF occurred via suitable signaling (e.g., X2 or S1).
Solution 4: The fourth solution is sending the RLF report when returning to the LTE RAN in the case of a too late IRAT HO from the LTE RAN to the 2G/3G RAN and detecting the connection failure at the RNC of the 2G/3G RAN in the case of a too early IRAT HO from the 2G/3G RAN to the LTE RAN. Solution 4 for Scenario 1 is illustrated in FIG. 14 and is the same as that for Solution 1, Scenario 1. For Solution 4, Scenarios 2a and 2b, the UE does not report the connection failure to the network. Rather, the RNC of the 2G/3G network can understand that the UE was previously camped on the 2G/3G network and is returning to the 2G/3G network after a connection failure during an IRAT HO to the LTE RAN.
As discussed in the Detailed Description below in detail, the inventors have found that the solutions for obtaining the RLF reports from the UEs in the various scenarios discussed above give rise to new issues with respect to delayed RLF reporting. As such, there is a need for systems and methods that address these new issues.